1. Field of the Invention
The present invention relates to a printing apparatus and a density variation correction method and more particularly to a printing apparatus and a density variation correction method which optically detect density variations and, based on the result of detection, perform a density variation correction.
2. Description of the Related Art
With the widespread use of information equipment in recent years, the use of printing apparatus, the peripheral devices of the information equipment, is also spreading quickly. Among the printing systems there are a wire-dot system, a thermosensitive system, a heat transfer system, and an ink jet system. Because of the advantages of low noise, low running cost, small size, and ease with which color inks can be introduced, the ink jet system in particular has found a wide range of applications including printer, facsimile and copying machine.
In a print head of a serial type ink jet system, for example, a plurality of nozzles are arranged in a direction perpendicular to a scan direction of the print head. Ink droplets are ejected from these nozzles to form an image.
However, the nozzles often have differing ejection characteristics, including the amount of ink ejected and the ink ejection speed, due to parts tolerances, variations in manufacturing processes, or changes with the passage of time. Increased ejection characteristic variations lead to density variations, resulting in banding and striped variations, significantly degrading the quality of a formed image.
The striped variations are density variations in the form of stripes extending in the main scan direction which in many cases appear periodically and therefore are very conspicuous, badly deteriorating the image quality. There are the following possible factors for such striped variations. In a so-called multinozzle type printing unit with a number of ink nozzles, in which a heater (electrothermal transducer) is installed in each ink passage communicating with the corresponding nozzle to produce heat energy for ejecting ink, the following factors may be listed as the possible causes for the striped variations.
(1) Variations in the amount of ejected ink and in the ejection direction caused by variations in the size of heaters and nozzles;
(2) Deviations between the feed of the print medium and the print width in the serial scan system;
(3) Differences in an ink density change between differing print times; and
(4) Movement of ink on the print medium.
A variety of methods have been proposed to prevent the striped density variations to enhance image quality.
For example, Japanese Patent Application No. 59-31949 (1984) discloses a method which, when the print unit of a serial scan system repeats the scan operation in the main scan direction to print one line of an image at one time, prevents striped variations from being formed at a joint between adjacent lines of print areas. This method overlaps the lowermost end of the preceding line of print area and the uppermost end of the next line of print area, with the image at the joined portion between the two print areas completed by two scans.
Another method for enhancing the image quality by eliminating striped variations is a divided printing method (multipass printing method) which completes one print area on the print medium by scanning the print unit over the area a plurality of times. This divided printing method is effective in eliminating the striped variations. To produce a sufficient effect of this method, however, the number of scans of the print unit over one print area, i.e., the number of divisions, needs to be increased, which in turn leads to an increased throughput.
A method for suppressing the striped variations without using the divided printing method is, for example, a head shading method such as described in Japanese Patent Application Laid-Open No. 5-69545 (1993).
This method performs as follows. First, the print unit prints a predetermined test pattern for determining a correction value on the print medium. The density of the printed test pattern is read one line at a time by a scanner with solid-state image sensors such CCDs. Then, the read image is position-corrected properly, after which the densities of individual lines of the image are allocated to the rasters corresponding to the nozzles of the print unit. Changes in the density of the printed image are caused by errors in the ink ejection amount among the nozzles, the deviations of ink ejection direction, or the spreading of ink over the print medium.
Next, from the density data corresponding to the individual rasters, the correction value of print density is determined for each nozzle. Then, based on the correction values, a xcex3 table or a drive table for individual nozzles is modified to change the amount of ink to be ejected. These corrections include such a density correction as an output xcex3 correction which lowers the density of the rasters that print darker than desired when no correction is made. For the rasters that print lighter than desired when no correction is made, the density correction such as output xcex3 correction is performed to increase the density of these rasters, thereby reducing the density variations (striped variations).
An example method using an input device such as a scanner is disclosed in Japanese Patent Application Laid-Open No. 1-41375 (1989). This method involves printing a patch pattern for each of cyan (C), magenta (M), yellow (Y) and black (K) inks, reading these patch patterns with a scanner incorporating image sensors such as CCDs, detecting a deviation between the density value thus read and a density value expected of each patch pattern and, based on the detected deviation, correcting the density value of image data. The CCDs used in the scanner have almost the same resolution as the density of the dots forming the printed patch and thus can read the density in units of dot. It is therefore possible to make corrections in units of nozzle corresponding to each dot.
In the conventional technology described above that corrects the density based on the read data of the test pattern, however, the density of the test pattern is read one line or one dot at a time by an expensive scanner using CCDs. It is difficult to assume that all users of the printing apparatus have such an expensive scanner. Therefore, the printing method capable of the above-described density correction is considered inappropriate for personal users.
Because the test pattern is read one line or one dot at a time depending on the scanner used, the reading takes a large amount of time. Further, an additional function is required to calculate the correction value of the print density from the read data of the test pattern.
Further, when the printing apparatus is fitted integrally with a test pattern reading scanner, the overall size and cost of the apparatus will increase.
An object of the present invention is to solve these problems, i.e., to provide a test pattern printing method, an information processing apparatus, a printing apparatus and a density variation correction method, all capable of obtaining output characteristics of a print unit and determining a correction value for output density.
In a first aspect of the present invention, there is provided a printing apparatus for performing a printing operation with a print head having a plurality of print elements, comprising:
an optical sensor having a light emitting portion and a light receiving portion;
pattern forming means for printing on a print medium a plurality of predetermined patterns conforming to a light emitting wavelength range of the optical sensor, each of the plurality of patterns being formed by each corresponding print element or each corresponding block made up of a plurality of print elements;
measuring means for emitting light from the light emitting portion of the optical sensor against the patterns printed on the print medium by the pattern forming means and measuring optical characteristics of the plurality of patterns; and
correction means for correcting image data to be used by the print head according to the optical characteristics measured by the measuring means.
In a second aspect of the present invention, there is provided a density variation correction method using a printing apparatus, the printing apparatus performing a printing operation by using a print head having a plurality of print elements, the correction method comprising:
a step of using an optical sensor having a light emitting portion and a light receiving portion; a pattern forming step for printing on a print medium a plurality of predetermined patterns conforming to a light emitting wavelength range of the optical sensor, each of the plurality of patterns being formed by each corresponding print element or each corresponding block made up of a plurality of print elements;
a measuring step for emitting light from the light emitting portion of the optical sensor against the patterns printed on the print medium by the pattern forming step and measuring optical characteristics of the plurality of patterns; and
a correction step for correcting image data to be used by the print head according to the optical characteristics measured by the measuring step.
In a third aspect of the present invention, there is provided a test pattern printing method for printing on a predetermined print medium a test pattern whose density is optically detected by a density sensor to obtain output characteristic information on a plurality of nozzles provided in a print unit mounted on the printing apparatus, the test pattern printing method comprising the steps of:
dividing a nozzle array made up of a plurality of nozzles provided in the print unit into a plurality of nozzle blocks; and
printing each of patches in a size and shape that enables the density of the patch to be optically detected by the density sensor by using only the nozzles of the same nozzle block allocated to the patch being printed;
wherein the test pattern comprises a plurality of patches.
In a fourth aspect of the present invention, there is provided an information processing apparatus for printing on a predetermined print medium a test pattern whose density is optically detected by a density sensor to obtain output characteristic information on a plurality of nozzles provided in a print unit mounted on a printing apparatus, the information processing apparatus comprising:
a means for dividing a nozzle array made up of a plurality of nozzles provided in the print unit into a plurality of nozzle blocks; and
a means for printing each of patches in a size and shape that enables the density of the patch to be optically detected by the density sensor by using only the nozzles of the same nozzle block allocated to the patch being printed;
wherein the test pattern comprises a plurality of patches.
In a fifth aspect of the present invention, there is provided a printing apparatus for printing on a predetermined print medium a test pattern whose density is optically detected by a density sensor to obtain output characteristic information on a plurality of nozzles provided in a print unit mounted on the printing apparatus, the printing apparatus comprising:
a means for dividing a nozzle array made up of a plurality of nozzles provided in the print unit into a plurality of nozzle blocks; and
a means for printing each of patches on the print medium by using only the nozzles of the same nozzle block allocated to the patch being printed;
wherein the test pattern comprises a plurality of patches.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.